Rock Shox Recon RL noise

DanDare446
DanDare446 Posts: 7
edited August 2018 in MTB workshop & tech
I’ve just purchased a Bossnut Evo, upon carrying out initial sag, pre-load & rebound I’m getting a ‘suction, or sshhhh’ noise from the shocks if I get on the bike & use my weight to compress them, the noise is worse when the rebound is set at the slowest return rate & almost disappears when set for fastest (over sprung) rebound. This noise occurs on both down & up stroke.

I thought this wasn’t right so popped it in to Go Outdoors today, the cycle chap said it’s normal, I wasn’t convinced so asked if we could try another Evo which was in there for sale but this one done the same thing.

I can understand why there could be a noise of air going through a hole slowly thus causing the shock to rebound slowly but I’d like to know if this is in fact normal.

I’m taking it out on Saturday for some forest trail fun so I’ll have to see if it’s still doing it afterwards...

Thanks.

Comments

  • steve_sordy
    steve_sordy Posts: 2,443
    That is oil being sucked or forced through the holes in the system that provides the damping. The more damping, the more tortuous the route the oil has to take. This is why the more rebound damping, the worse the noise. The noise is air bubbles in the oil. Air bubbles are more likely in budget suspension kit. There may be a cure for this, but I don't know what it may be.
  • The Rookie
    The Rookie Posts: 27,812
    The noise is cavitation, not air bubbles!
    https://en.wikipedia.org/wiki/Cavitation
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • JBA
    JBA Posts: 2,852
    The Rookie wrote:
    The noise is cavitation...

    Which creates air bubbles.

    DanDare446 - you could try letting all the air out of the fork and then repressurising it. That might reduce the problem.
    “Life has been unfaithful
    And it all promised so so much”

    Giant Trance 2 27.5 2016 ¦ Sonder Broken Road 2021¦ Giant Revolt Advanced 2 2019 ¦ Giant Toughtroad SLR 1 2019 ¦ Giant Anthem 3 2015 ¦ Specialized Myka Comp FSR 2009
  • steve_sordy
    steve_sordy Posts: 2,443
    I'm still not convinced that it is cavitation in the shock.

    The "bubbles" produced by cavitation are bubbles of near vacuum. They collapse quickly and can produce tremendous damage. Cavitation has been known to reduce ship's propellers to lace curtains. You don't want that inside a shock as it would quickly damage it beyond repair.

    Anyway, every day should be a learning day. And what I learned today is not to try to answer technical questions at almost midnight after an evening at the pub as I am less likely to spot my spelling mistakes. :roll:
  • The Rookie
    The Rookie Posts: 27,812
    JBA wrote:
    The Rookie wrote:
    The noise is cavitation...

    Which creates air bubbles.
    No, they are vacuum or vapour....if you'd read the provided link.....in the case of damper stacks they are vapour and non destructive.

    Letting the air out of a totally different space in the suspension clearly will have no effect!
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • DanDare446
    DanDare446 Posts: 7
    Well, thanks for your input all. I took it out on a 10 mile Singletrack route today, apart from it being way too hot & muggy the shock still made the same annoying noise all the way round. Interestingly, the Suntour Raidon air fork on my Bizango makes no such noise regardless of rebound position.

    Can’t say I’m completely happy, I’m out on it again for another 10 mile Singletrack loop elsewhere tomorrow so I’ll ‘re-assess’ it then. On top of this, the front end has a rattle over bumps which I can’t yet locate.
  • The Rookie
    The Rookie Posts: 27,812
    Are you running a lot of rebound damping?
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • DanDare446
    DanDare446 Posts: 7
    The Rookie wrote:
    Are you running a lot of rebound damping?

    On the front, with 0 ‘clicks’ being minimum damping & 5 maximum, I set it to 2.

    (On the rear, 0 to 9 I set it to 5)

    This is what felt good on the trail I was on yesterday...
  • The Rookie
    The Rookie Posts: 27,812
    That should be fine on the rebound, its just that more rebound will increase the noise as well as too much giving a rattling.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.

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